Originally published in 1971 as A Guide to Middle-Earth, before the publication of The Silmarillion, the first edition contained only information from The Lord of the Rings and The Hobbit. In 1978, a new edition (The Complete Guide to Middle-earth: from The Hobbit to The Silmarillion), containing material from Silmarillion, was published.

Foster relates the Elvish words ambar "world" and umbar "fate". In the entry of Ambar, he mentions it is a concept related to fate of the world. The Etymologies showed that the two have different roots; a distinct word ambar is actually linked to the root for "home".